This invention relates to raster distortion correction correction for a deflection circuit.
In a television receiver, the electron beams inside the television receiver picture tube are deflected by magnetic fields generated by sawtooth deflection currents flowing in horizontal and vertical deflection windings. The deflected electron beams scan a raster pattern on the picture tube phosphor screen. The raster pattern, without correction, may display various geometric distortions such as side or east-west pincushion distortion. The correction of inner raster distortion requires modulation of the S-shaping as a function of the distance along the vertical axis of the raster display, i.e. from top to the center and from the center to the bottom of the raster. This is achieved by modulation of the voltage across the S-capacitor. The modulated voltage appears also across the horizontal deflection winding and causes a modulation of the deflection current.
To correct side pincushion distortion, the peak-to-peak horizontal deflection trace current in the horizontal deflection winding is modulated at a vertical rate in a parabolic manner. Such parabolic modulation may be accomplished in a horizontal deflection circuit that includes an East-West modulator. Examples of East-West modulators that provide both side pincushion distortion correction and inner raster distortion correction are described in, for example, U.S. Pat. No. 3,906,305, entitled, CIRCUIT ARRANGEMENT FOR GENERATING A SAWTOOTH DEFLECTION CURRENT THROUGH A LINE DEFLECTION COIL, in the name of Nillesen (the Nillesen Patent).
An East-West modulator circuit, embodying an inventive feature, includes an amplifier for generating a voltage across a capacitor at a vertical rate in a parabolic manner. The capacitor voltage is coupled via an inductor to the deflection circuit for providing pincushion correction. The amplifier includes a transistor stage having a main current conducting path which varies the voltage across the capacitor.
The current through the inductor and the transistor of the modulator could include an undesirable decaying oscillating component or ringing following a fast transition in the input voltage to the amplifier, resulting in a width ringing at the top of the picture. It may be desirable to dampen the ringing in the deflection current by dampening the ringing in the capacitor.
In carrying out an inventive feature, a current feedback path is formed in the amplifier. Consequently, an output impedance of the amplifier at a junction terminal between the capacitor and the inductor sufficiently increases to dampen the ringing and reduce picture distortion.
A deflection apparatus embodying an inventive feature includes a deflection winding and a source of a synchronizing input signal at a frequency related to a first deflection frequency. A switching semiconductor is responsive to the first input signal and is coupled to the deflection winding for generating a deflection current in the deflection winding. A retrace capacitance is coupled to the deflection winding to form a retrace resonant circuit with the deflection winding during a retrace interval of a is deflection cycle. A source of a modulation input signal at a frequency related to a second deflection frequency is provided. A modulation amplifier is responsive to the modulation input signal for producing, from a current produced in an output stage of the modulation amplifier, a modulation voltage having a frequency related to the second deflection frequency to module the deflection current in a manner to provide side raster distortion correction. The modulation amplifier stage includes both a current feedback path that increases an output impedance of the output stage and a voltage feedback path that decreases the output impedance.